They run "B" impellers to get more top end acceleration.
The lower the letter, the stiffer the impeller, and the more load is placed on the engine. A "B" impeller requires less HP because it doesn't load the engine as much, so it's easier for the motor to spin and easier to get your RPMs up, which will give you better top end acceleration response. Whereas the stiffer impeller, while yes it does move more water, it also puts more of a load on the engine, which makes the engine have to make more torque to spin it, which makes the engine spin the pump harder, and gives you more pulling power for when you're pulling skiiers.
Acceleration is all in the RPMs. Lower RPMs=more torque/higher RPMs=more HP.